The present invention relates to an apparatus and method for controlling an electrolytic roughening treatment of a material, such as a metal plate, and more particularly it relates to an apparatus and method of controlling the electrolytic reaction during an electrolytic roughening treatment a metal plate.
It is known to use an aluminum plate (including an aluminum alloy) as a support for a printing plate such as a support for an offset printing plate. In such cases, a suitable adhesion and a suitable amount of water between the surface of the aluminum plate and a photosensitive layer is necessary. To achieve this, the surface of the aluminum plate must be uniform and finely roughened. If the aluminum plate is not finely roughened, both the printing performance and durability of the printing plate will be highly affected. Accordingly, it is important to achieve a satisfactory roughening of the plate during the manufacture thereof.
In general, the roughening of an aluminum support of a printing plate involves the use of an alternating electrolytic etching method which, in turn, involves the use of an alternating current (e.g., a sine waveform, a square waveform, a special alternating waveform, etc.). The roughening treatment of the aluminum plate is performed by using a graphite electrode or the like disposed adjacent to the aluminum plate for applying the alternating current. The plate is usually only roughened once. As a result, the depth of pits formed by the roughening process in the plate is small over the whole surface thereof and the durability of the roughened printing plate for printing applications will deteriorate. Therefore, in order to obtain a uniformly and closely roughened aluminum plate having deep pits as compared with their diameters, a variety of methods are proposed. A first method is disclosed by Japanese Patent Laid-Open No. Sho. 53-67507, wherein a roughening method uses a current of particular waveform for an electrolytic source. Japanese Patent Laid-Open No. Sho. 54-65607 discloses another method which controls the ratio between an electrical quantity during a positive period and during a negative period at the time of alternating electrolytic roughening. Still another method is to control the waveform supplied from the electrolytic source (Japanese Patent Laid-Open No. Sho 55-25381). Finally, another method is directed to controlling the combination of current density (Japanese Patent Laid-Open No. Sho 56-29699).
Japanese Patent Examined Publication No. Sho 61-60797, discloses a uniform roughened surface as a result of supplying an alternating current to the aluminum plate, in which at least one of the positive periods and negative periods includes a rest period of 0 Volt, so that the electricity quantity of the positive period may be larger than that of the negative period.
However, when the aluminum plate is composed of an alloy, containing many ingredients (such as JIS3003 material), having an irregular yield of a small amount ingredients among the aluminum lots, it is likely to transform the roughened form and to change the printing performance.
As a method to eliminate the above and other problems, particularly when using an alternating waveform, the inventor of the present invention provides a method (assuming that t.sub.f is the positive period and t.sub.r is the negative period), of applying an alternating current between an aluminum support and an electrode. The alternating current includes periods, in which the current reaches peak levels in both the positive and negative periods t.sub.f and t.sub.r, respectively, adjusted in the range of 0.1-20% of either period, thereby shortening the period it takes to reach the peak levels. This method then enables mass production and obtains a uniform roughening treatment, when using a power circuit including an inductive component larger than an inductive component of a load and a circuit to generate an alternating current by a current inverting control circuit. (Japanese Patent Laid-Open No. Hei. 3-82799).
However, in a waveform controlled by the method of Japanese Patent Laid-Open No. Hei. 3-82799, when the current direction in the load changes, large electric power is required, in which a top line having a trapezoidal form of the output waveform develops a tendency to incline upward and rightward. As a result, pits are formed in the period develop some distributing ranges.
Although the method disclosed in the publication satisfies the requirements of uniform pits, more satisfactory printing performance and more uniform pits are recently required and can be achieved using the invention as described below.